In recent years, a light emission efficiency of a light-emitting diode device has been extremely improved, and an application of the luminous diode device to an illumination is being progressed. In particular, in the case in which there is used a light emitting diode device as a back light for a liquid crystal display, a satisfactory gamut of reproducible colors and a high speed response can be implemented and it is expected that a high quality display be achieved.
Conventionally, the main stream of such a back light for a liquid crystal display has been the so-called edge light type in which a cold cathode tube as an light source is disposed on the edge face of the chassis for thinning and low power consumption of the apparatus. However, a demand of enlarging a liquid crystal display has been increased in recent years, and the edge light type has a limitation in the case in improving a luminance and a uniformity in the luminance. Therefore, an adoption of a direct lighting type light has been examined for a large size liquid crystal display.
FIG. 12 is a cross sectional view for showing a configuration of a conventional flat light source device of a direct lighting type used in a liquid crystal display (TECHNO-FRONTIER SYMPOSIUM 2005, Thermal Design and Countermeasure Technology Symposium, Issue Date: Apr. 20, 2005 (Japan Management Association), Session G3: Latest Design Case of Heat Radiation Mounting I (pp. G3-3-1 to G3-3-4), and so on). The flat light source device 101 is disposed directly below a liquid crystal panel 21. In the flat light source device 101, LED light sources 5 using a light emitting diode device are disposed in an array pattern on a bottom face of a chassis 12. Moreover, the bottom face and side faces of the chassis 12 are covered with a reflecting sheet 13. Furthermore, over the LED light source 5, a diffusing sheet 14 and a prism sheet 15 are disposed in the range of 1 to 5 cm in general apart from the LED light source 5.
In the case in which a light is emitted from the LED light source 5, the emitted light travels directly toward the diffusing sheet 14, or is reflected by the reflecting sheet 13 and travels toward the diffusing sheet 14. The emitted light is then irregularly reflected in the diffusing sheet 14, and is inclined in a vertical direction by passing through the prism sheet 15. The emitted light then enters a liquid crystal panel 21. Lights emitted from different LED light source 5 are mixed in a space between the LED light sources and the diffusing sheet 14. The mixing is then improved by an irregular reflection in the diffusing sheet 14, thereby implementing a uniform luminance and a uniform chromaticity. Moreover, a luminance at the section directly over the LED light sources 5 is higher than that of other sections. Therefore, a uniformity in a luminance can be further improved by increasing a diffusability of the diffusing sheet 14 at the section directly over the LED light sources 5, thereby improving a uniformity of a luminance of the entire liquid crystal panel 21.
In the conventional flat light source device, the diffusing sheet is disposed and the diffusing sheet is apart from the LED light sources in order to uniform a luminance and a chromaticity as described above. However, in the case in which a color mixture is carried out by using LED light sources of a plurality of colors such as RGB, a color mixture is insufficient and unevenness of colors may be found. In addition, there is a problem that a luminance of the section directly over the LED light sources becomes higher.
Accordingly, to reduce an unevenness of luminance and the like, a luminance of the section directly over the LED light sources 5 is reduced by increasing a diffusability of the diffusing sheet 14 directly over the LED light source 5, or a so-called lighting curtain for improving a uniformity of a luminance of the entire liquid crystal panel 21 is disposed directly over the LED light sources in order to reduce a luminance of the section directly over the LED light sources in some cases. However, the above means cause a utilization efficiency of a light to be lowered.
Moreover, in the case in which the diffusing sheet is made further apart from the LED light sources, a nonuniformity in chromaticity and a nonuniformity in luminance can be reduced. However, this method causes a thickness of a back light to be enlarged, and is not preferable for a flat panel display.
In order to implement a uniform chromaticity and a uniform luminance and suppress a thickness of the flat light source device, it is effective to use a light guide plate to shut up and propagate a light emitted from a linear type light source or a dot type light source in the light guide plate, and to emit a uniform light forward from the light guide plate (Japanese Patent Publication No. 3151830).
In the above described flat light source device of a direct lighting type, in order to sufficiently implement a uniform chromaticity and a uniform luminance and suppress a thickness of the planar light source device, it is effective to dispose a light guide plate that faces to LED light sources in front of a substrate in which a plurality of LED light sources is disposed and to dispose a reflecting layer on the back face side of the light guide plate if necessary, thereby propagating a light that has been emitted from the LED light source and has traveled to the back face in the light guide plate and then extracting the light from the front face of the light guide plate.
However, even in the case in which such a light guide plate is used, in the case in which LED light sources of a plurality of colors such as RGB are used, a nonuniformity in chromaticity may be found. In addition, a luminance of the light guide plate at the position directly over the LED light sources becomes higher, and a nonuniformity in luminance cannot be canceled to a sufficient level.
As effective technology for canceling a nonuniformity in luminance, Japanese Laid-Open Patent Publication No. 2003-8081, Japanese Laid-Open Patent Publication No. 2003-8068, and Japanese Laid-Open Patent Publication No. 2004-133391 disclose a lens for changing a direction of a light emitted from an LED light source to a horizontal direction. As shown in FIG. 13(a), a lens 31 is provided with a depression 32 in a funnel shape on the upper face and a saw tooth shaped portion 33 on the periphery. A light emitted from an LED light source 5 disposed below the lens 31 is reflected and refracted at the funnel shaped portion 32 and the saw tooth shaped portion 33, thereby spreading an irradiation region of a light in a horizontal direction and suppressing an increase in a luminance at the position directly over the LED light source 5.
Although the lens 31 can guide most lights emitted from the LED light source 5 in a horizontal direction, the shape of the lens is complicated. On the other hand, as shown in FIG. 13(b), while a lens 41 provided with a conical depression 42 on the upper face of a cylindrical substrate has a simple shape, part of lights emitted from the LED light source 5 is reflected at a side face 43 of the cylinder and travels upward through the lens. The light that travels upward through the lens may cause a nonuniformity in chromaticity.